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Home / DTC / B1A03 — KOS key4 internal error

B1A03 — KOS key4 internal error

Detailed page for trouble code B1A03.

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B1A03

MITSUBISHI B — Body

KOS key4 internal error

Brand: MITSUBISHI
Type: B — Body
AI status
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ready
Completed 100%
Page language: EN

Causes

  • Corrupted key transponder data for key4 (stored key ID).
  • Weak or dead battery in the key fob.
  • Faulty KOS/immobilizer antenna (coil) or its wiring/connectors near ignition/key reader.
  • Intermittent or lost communication between KOS module and body/engine control modules (CAN/LIN wiring).
  • Internal fault in the KOS/immobilizer control unit (software or hardware).
  • Failed or incomplete key registration/programming.

Symptoms

  • Key not recognized (key message displayed or indicator light).
  • Inability to start the engine or intermittent cranking/no-crank when using affected key.
  • Security/immobilizer warning lamp illuminated or flashing.
  • Other keys may work normally while key4 fails (if multiple keys present).
  • Remote keyless functions (lock/unlock, push-button start) may be inconsistent for that key.

What to check

  • Read and record all stored codes and freeze frame data with a capable diagnostic scanner (body/immobilizer module).
  • Verify which key(s) fail: try known-good key(s) and note difference in behavior.
  • Check key fob battery voltage and replace with a fresh battery if low.
  • Visually inspect antenna/reader area, connectors, wiring harness, and module connectors for corrosion, damage, or water intrusion.
  • Check fuses and power/ground at the KOS/immobilizer module.
  • Confirm vehicle software/ECU versions and any Technical Service Bulletins for KOS/immobilizer.

Signal parameters

  • Key transponder communication: LF or HF RFID type depending on model (commonly 125 kHz LF / 13.56 MHz HF for smart keys) — verify exact frequency in service manual.
  • KOS module supply: nominal 12 V battery feed and chassis ground — verify presence and stability during tests.
  • Antenna/reader coil: passive RFID coil interface — resistance and waveform should be checked per factory specs (consult service manual for expected ohms and test procedure).
  • Data communications: CAN or LIN messages between immobilizer/KOS and BCM/ECM — check for valid message traffic with scan tool/sniffer.

Diagnostic algorithm

  1. Retrieve and record all codes from the KOS/immobilizer and related modules (BCM/ECM). Note conditions and occurrence counts.
  2. Confirm vehicle battery voltage is healthy (≥12.4 V) and fuses for immobilizer/KOS are OK.
  3. Test the suspect key: replace the key fob battery and retry. If available, try a known-good key to see if problem follows the key or the vehicle.
  4. Visually inspect antenna/reader, wiring and connectors for damage, corrosion or water ingress. Wiggle-test connectors while monitoring fault status if safe.
  5. Use a diagnostic tool to attempt to read key ID/transponder data. If the key ID cannot be read for key4 but other keys read fine, suspect the key or its transponder.
  6. Check module power and ground at the KOS unit with multimeter. Back-probe connectors for stable 12 V and good ground during operation.
  7. Monitor CAN/LIN traffic (if supported) to verify communication between KOS and engine/body modules; look for lost-communication or invalid data messages.
  8. If hardware and wiring appear good, attempt re-registration/reprogramming of key4 following factory procedure. Clear codes and retest.
  9. If code persists after key reprogramming and wiring checks, consider module software update (consult dealer/service bulletins) or replacement of the KOS/immobilizer module. After replacement, perform key programming and full system relearn.
  10. After any repair, clear stored codes, retest all keys and drive/test to confirm no recurrence.

Likely causes

  • Key battery weak or dead causing poor transponder interrogation.
  • Damaged or corroded antenna coil connector near reader (most common in practice).
  • Stored key data corrupted (requires re-registration) or key physically damaged.
  • Intermittent module supply (low battery voltage, loose ground or fuse).
  • Internal KOS module hardware or firmware malfunction (less common).

Fault status

⚠️ Status
KOS key4 internal error: The Keyless Operation System has detected an internal fault related to stored key slot 4 or its transponder/communication path. The system may not reliably recognize that key and immobilizer protection may prevent normal starting or remote functions. Further diagnosis required to isolate key, antenna, wiring, or module fault.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

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406

Browse 406 MITSUBISHI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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